Recently, the demand on the high-speed running stability of the vehicle becomes further severer, and hence it is strongly desired to use rubber having excellent wet-skid resistance and dry gripping property in addition to the wear resistance and fracture properties required from the past as a rubber material for a tire tread in the vehicle. However, it is difficult to simultaneously satisfy these properties, and particularly the wear resistance, wet-skid resistance and dry gripping property have a conflicting relation to each other. The gripping property is dependent on the hysteresis loss of the rubber composition. In order to enhance the gripping property of the rubber composition, a greater amount of an aromatic type softening agent has hitherto been compounded, which brings about the lowering of the fracture properties.
In order to solve this problem, the invention disclosed in JP-A-63-101440 proposes the use of low molecular weight SBR. However, a double bond having a crosslinking property is existent though the molecular weight is low, so that there is a problem that a part of the low molecular weight component is crosslinked with rubber matrix and taken into the matrix and hence the sufficient hysteresis loss is not produced. Also, if the double bond part is rendered into a saturated bond by hydrogenation so as not to take the low molecular weight component into the matrix through the crosslinking, the compatibility with the matrix becomes considerably poor, and as a result, there are problems that the fracture properties lower and the low molecular weight component bleeds out.
Furthermore, rubber compositions aiming at the improvement of workability, low fuel consumption, fracture properties and gripping force by a blend of high molecular weight body and a low molecular weight body are disclosed in the inventions of JP-B-59-52664, JP-A-58-147442, JP-A-58-147443, JP-A-60-240746, JP-A-61-203145, JP-A-62-135506, JP-A-64-16845 and the like, but they can not sufficiently satisfy the wet-skid resistance, dry gripping property and wear resistance.
JP-A-2000-129037 discloses a technique solving the above problems. This technique uses a rubber composition using a specified styrene-butadiene copolymer (A′) and a styrene-butadiene copolymer having a specified hydrogenation ratio (B′) as a rubber component, in which a bound styrene content of the copolymer (B′) is made larger by a given value than a bound styrene content of the copolymer (A′), as a rubber material for a tire tread. However, tires having a further improved gripping property are demanded at the present, and the gripping property is insufficient even in the tire using the rubber composition disclosed in the JP-A=2000-129037 in the tread.